System and method for detecting abnormal passenger behavior in autonomous vehicles

ABSTRACT

A method and system are disclosed for monitoring passengers in within a cabin of a vehicle and determining whether the passengers are engaging in abnormal behavior. The method and system uses a novel vector to robustly and numerically represent the activity of the passengers in a respective frame, which is referred to herein as an “activity vector.” Additionally, a Gaussian Mixture Model is utilized by the method and system to distinguish between normal and abnormal passenger behavior. Cluster components of the Gaussian Mixture Model are advantageously learned using an unsupervised approach in which training data is not labeled or annotated to indicate normal and abnormal passenger behavior. In this way, the Gaussian Mixture Model can be trained at a very low cost.

This application is a continuation application of U.S. patentapplication Ser. No. 16/716,580, filed on Dec. 17, 2019, the disclosureof which is hereby incorporated herein by reference in its entirety.

FIELD

The device and method disclosed in this document relates to in-vehiclesensing systems and, more particularly, to detecting abnormal passengerbehavior in autonomous vehicles.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to the prior art by inclusion in this section.

In the near future, driverless cars, such as autonomous taxis operatedfor on-demand mobility service, will play an important role intransportation. It will soon become common practice for passengers, whoare strangers to each other, to share an autonomous taxi. Unlike atraditional taxi, where a driver can supervise the passengers,autonomous taxis will need surveillance systems to monitor the safety ofpassengers. Any abnormal behaviors of passengers, such as violentactivities, should be detected and monitored for its prevention.Accordingly, it would be beneficial to provide a monitoring system formonitoring passengers within the cabin of an autonomous vehicle andintelligently detect abnormal passenger behavior.

SUMMARY

A method for detecting abnormal passenger behavior in a vehicle isdisclosed. The method comprises receiving, with a processing system, afirst image frame of at least one passenger in a cabin of the vehiclefrom an image sensor. The method further comprises determining, with theprocessing system, based on the first image frame, a first numericalvector representing a pose and a motion of the at least one passenger inthe first image frame. The method further comprises detecting, with theprocessing system, based on the first numerical vector, abnormalpassenger behavior in the first image frame using a mixture model havinga plurality of cluster components representing normal passengerbehaviors.

A system for detecting abnormal passenger behavior in a vehicle isdisclosed. The system comprises an image sensor configured to generateand output image frames of at least one passenger in a cabin of thevehicle. The system further comprises a processing system operablyconnected to the image sensor and including at least one processor. Theprocessing system is configured to receive a first image frame from theimage sensor. The processing system is further configured to determine,based on the first image frame, a first numerical vector representing apose and a motion of the at least one passenger in the first imageframe. The processing system is further configured to detect, based onthe first numerical vector, abnormal passenger behavior in the firstimage frame using a mixture model having a plurality of clustercomponents representing normal passenger behaviors.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of the method and system areexplained in the following description, taken in connection with theaccompanying drawings.

FIG. 1 is a schematic top view of a vehicle with a cabin monitoringsystem.

FIG. 2 is a schematic view of components of the vehicle and the cabinmonitoring system of FIG. 1.

FIG. 3 shows a logical flow diagram for a method of detecting abnormalpassenger behavior in a cabin of a vehicle.

FIG. 4 shows a logical flow diagram for a method of deriving an activityvector for an image frame.

FIG. 5 shows an exemplary image frame in which two passengers are ridingin the back seat of the vehicle.

FIG. 6 shows an exemplary sequence of five image frames in which twopassengers are riding in the back seat of the vehicle.

FIG. 7 shows a further exemplary image frame in which a passenger ispushing another passenger.

FIG. 8 shows a graph illustrating the activity vector calculated basedon the exemplary image frame of FIG. 7.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the disclosure is therebyintended. It is further understood that the present disclosure includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the disclosure aswould normally occur to one skilled in the art which this disclosurepertains.

System Overview

With reference to FIGS. 1-2 an exemplary embodiment of a vehicle 100having a cabin monitoring system 104 is disclosed. The cabin monitoringsystem 104 is advantageously configured to monitor passengers in withina cabin 108 of the vehicle 100 and determine whether the passengers areengaging in abnormal behavior. In addition to the cabin monitoringsystem 104, the vehicle 100 includes a vehicle electronic control unit(“ECU”) 112 configured to operate a drive system 116, as well as variouselectronics of the vehicle aside from the cabin monitoring system 104,such as lights, locks, speakers, displays, etc. The drive system 116 ofthe vehicle 100 includes a drive motor, for example an internalcombustion engine and/or one or more electric motors, that drives thewheels of the vehicle 100, and the steering and braking components thatenable the vehicle 100 to be moved in a controlled manner.

In the illustrated embodiment of FIG. 1, the vehicle 100 is the form ofan automobile. However, in other embodiments, the vehicle 100 mayinclude any number of types of vessels having one or more cabins 108 formoving people, such as trains, buses, subways, aircraft, helicopters,passenger drones, submarines, elevators, and passenger moving pods. Thecabin 108 (which may also be referred to herein as a compartment) is atypically closed room for accommodating passengers. Although the vehicle100 is illustrated as having a single cabin 108, it will be appreciatedthat the vehicle 100 may include any number of individual and separatecabins 108 (e.g., multiple compartments or rooms inside a train car). Inthe illustrated embodiment, the cabin 108 includes four seats 120, 122,124, 126 in which passengers can be seated. However, the cabin 108 mayinclude more or less seats depending on the configuration and type ofthe vehicle 100. The vehicle 100 also includes one or more doors (notshown) enabling passengers to access the cabin 108 and the seats120-126. In addition, the vehicle 100 may include a rear hatch (notshown) enabling a user to access a cargo storage area of the vehicle100, for example a trunk or storage space behind the rear seats 124,126.

In at least one embodiment, the vehicle 100 is a shared autonomousautomobile that is configured to provide autonomous transportationservices in which the vehicle 100 drives autonomously to the location ofa passenger and then, upon the passenger entering the vehicle 100,autonomously transports the passenger to a desired location using thepublic roadway network. The passenger may engage the autonomoustransportation services of the vehicle 100 using a smartphone or smartdevice application (i.e. an “app”), for example. The passenger is alsoreferred to herein as an occupant, a user, an operator, or a person. Inother embodiments, the vehicle 100 is any type of passenger vehicle, asdescribed above, and, in some embodiments, may be occupant controlled orremotely controlled.

The cabin monitoring system 104 comprises a vehicle computer 130 that isoperably connected to one or more image sensors 134, 138 that arearranged throughout the vehicle. The image sensors 134, 138 may be videoor still image RGB cameras, each of which has, for example, acharge-coupled device (CCD) or an active-pixel sensor for generatingdigital image data in the form of image frames. In other embodiments,the image sensors 134, 138 may include thermal or infrared sensors, aradar imaging system, a LIDAR imaging system, or another suitableimaging system.

In the illustrated embodiment, the cabin monitoring system 104 includestwo interior image sensors 134, 138 arranged within the cabin 108 andconfigured to generate an image of a portion of the cabin 108. In oneembodiment, the interior image sensors 134, 138 are arranged in or onthe roof of the vehicle 100 and directed downwardly into the cabin 108toward the respective seat or seats 120-126 for imaging. In otherembodiments, the interior image sensors 134, 138 may be arranged in theseats or in the dash of the vehicle 100. For example, in one particularembodiment, the image sensors for imaging the front seats 120, 122 arearranged in the dash of the vehicle 100, while the image sensors forimaging the rear seats 124, 126 are arranged in the front seat 120, 122that is directly in front of the respective rear seat 124, 126. In someembodiments, additional exterior image sensors (not shown) may bearranged on an exterior of the vehicle 100 so as to generate an image ofa portion of the exterior of the vehicle 100.

In the illustrated embodiment, the front image sensor 134 generatesdigital image data of the front of the cabin, including the front seats120, 122, and the rear image sensor 138 generates digital image data ofthe rear of the cabin 108, including the rear seats 124, 126. In otherembodiments, the cabin monitoring system 104 may include a single imagesensor that captures images of the entire cabin 108, including all ofthe seats 120-126, a separate image sensor directed at each of the seats120-126, or any desired configuration of image sensors to generatedigital images of each seat in the vehicle.

The vehicle computer 130 is configured to process image data receivedfrom one or more of the image sensors 134, 138 to monitor passengers inwithin the cabin 108 of the vehicle 100 and determine whether thepassengers are engaging in abnormal behavior. The vehicle computer 130may additional be configured to perform other complex tasks such asautonomous navigation of the vehicle 100 and interfacing with thepassengers or a smartphone in the possession of the passenger to provideautonomous transportation of the passenger.

With reference now to FIG. 2, exemplary components of the vehiclecomputer 130 of the cabin monitoring system 104 are described. In theillustrated embodiment, the vehicle computer 130 comprises at least aprocessor 200 and associated memory 204. The memory 204 is configured tostore program instructions that, when executed by the processor 200,enable the vehicle computer 130 to perform various operations describedelsewhere herein, at least including monitoring passengers in within thecabin 108 of the vehicle 100 and determining whether the passengers areengaging in abnormal behavior. The memory 204 may be of any type ofdevice capable of storing information accessible by the processor 200,such as a memory card, ROM, RAM, hard drives, discs, flash memory, orany of various other computer-readable medium serving as data storagedevices, as will be recognized by those of ordinary skill in the art.Additionally, it will be recognized by those of ordinary skill in theart that a “processor” includes any hardware system, hardware mechanismor hardware component that processes data, signals or other information.The processor 200 may include a system with a central processing unit,graphics processing units, multiple processing units, dedicatedcircuitry for achieving functionality, programmable logic, or otherprocessing systems.

In the illustrated embodiment, the vehicle computer 130 further includesa communication interface 208 configured to enable the vehicle computer130 to communicate with the image sensors 134, 138 and with the vehicleECU 112 via one or more communication buses 142, which may take the formof one or more a controller area network (CAN) buses. The communicationinterface 212 may include physical terminals for connecting to wiredmedia (e.g., the communication buses 142). Additionally, thecommunication interface assembly 212 may include one or more modems, buscontrollers (e.g., a suitable CAN bus controller), or other suchhardware configured to enable communications with the image sensors 134,138 and the vehicle ECU 112.

In the illustrated embodiment, the vehicle computer 130 further includesone or more radio transceiver(s) 212 configured to communicate with aremote server (e.g., a cloud service), as well as with a smartphone orother smart device in the possession of the passenger, for the purposeof providing autonomous transportation services. The radiotransceivers(s) 212 may include transceivers configured to communicatewith the Internet via wireless telephony networks, such as Global Systemfor Mobiles (“GSM”) or Code Division Multiple Access (“CDMA”)transceivers. Additionally, the radio transceiver(s) 212 may include aBluetooth® or Wi-Fi transceiver configured to communicate locally with asmartphone or other smart device in the possession of the passenger.

As will be described in greater detail below, the memory 204 of thevehicle computer 130 stores program instructions corresponding to anabnormal behavior detection program 216. The abnormal behavior detectionprogram 216 includes program instructions and learned parameterscorresponding to a pose detection model 220 and to an activityclassification model 224. Additionally, the memory 204 stores image data228 including image frames received from the image sensors 134, 138, andactivity data 232 representing the activity of the passengers in eachimage frame.

Methods for Detecting Abnormal Passenger Behavior

The cabin monitoring system 104 is advantageously configured to monitorpassengers in within the cabin 108 of the vehicle 100 and determinewhether the passengers are engaging in abnormal behavior. Forexplanatory purposes only, it is noted that abnormal passenger behaviorsmay comprise violent behaviors such as arguing, fighting, grabbing,kicking, punching, pushing, or slapping, as well as non-violentbehaviors such as undressing. In contrast, normal passenger behavior maycomprise behaviors such as talking, touching, hugging, sitting still,drinking coffee, or crossing legs.

As will be discussed in greater detail below, the cabin monitoringsystem 104 uses a novel vector to robustly and numerically represent theactivity of the passengers in a respective frame, which is referred toherein as an “activity vector” for the respective image frame.Additionally, the activity classification model 224 includes a mixturemodel, in particular a Gaussian Mixture Model (GMM), utilized by thecabin monitoring system 104 to distinguish between normal and abnormalpassenger behavior. Particularly, based on training data in the form ofvideo of passengers riding in the cabin 108 of the vehicle 100, GaussianMixture Modeling is used to learn cluster components representingactivity vectors corresponding to normal passenger behavior. Thus, thecabin monitoring system 104 can determine whether the passengers areengaging in abnormal behavior by comparing activity vectors representingactual passenger behavior to the learned cluster components representingnormal passenger behavior. Thus, it will be appreciated that, as usedherein as it relates to the activity classification model 224 and/or themixture model thereof, the terms “abnormal behavior” or “abnormalpassenger behavior” merely refer to passenger behavior that is uncommonor rare in the training data and no particular qualitative orvalue-based meaning is ascribed to the terms.

Advantageously, an unsupervised approach can be utilized in which thetraining data is not labeled or annotated to indicate normal andabnormal passenger behavior. Particularly, because abnormal behaviors,such as violence, are generally rare, unannotated video of passengersriding in the cabin 108 of the vehicle 100 can be used to learn thecluster components representing normal passenger behavior. Thisunsupervised approach is advantageous because a large corpus of trainingdata can be collected and used for training at a very low cost.Additionally, because the definition of abnormal behavior, such asviolence, varies across individuals so the quality of the annotationswould be questionable in supervised approaches, which in turn wouldresult in poor performance. Moreover, since abnormal behavior, such asviolence, rarely occurs in practice, it would be hard to gather all thepossible abnormal behavior in training data in supervised approaches.Furthermore, supervised approaches tend to rely on heavily hand-craftedfeatures that may work well with the existing training data, but may notbe generalizable to detect future abnormal behavior when it is differentfrom the abnormal behavior in the training data.

FIG. 3 shows a logical flow diagram for a method 300 of detectingabnormal passenger behavior in a cabin of a vehicle. In the descriptionof the method 300, statements that a method, process, module, processor,system, or the like is performing some task or function refers to acontroller or processor (e.g., the processor 200) executing programmedinstructions (e.g., the program instructions 208) stored innon-transitory computer readable storage media (e.g., the memory 204)operatively connected to the controller or processor to manipulate dataor to operate one or more components in the cabin monitoring system 108and/or the vehicle 100 to perform the task or function. Additionally,the steps of the methods may be performed in any feasible chronologicalorder, regardless of the order shown in the figures or the order inwhich the steps are described. It will be appreciated that, in someembodiments, the operations of the processor 200 described herein can beperformed by other components of the vehicle 100 and/or of the cabinmonitoring system 108, such as the vehicle ECU 112 or integrated imageprocessors of the sensors 134, 138. Additionally, in some embodiments,the operations of the processor 200 described herein can be performed bya remote server, such as in cloud computing system.

The method 300 begins with a step of receiving an image frame andincrementing a frame count (block 310). Particularly, the processor 200of the vehicle computer 130 operates at least one of the image sensors134, 138 to receive a video feed consisting of a sequence of imageframes at a defined frame rate (e.g., 25 frames per second). In at leastone embodiment, the processor 200 stores the received image frames inthe memory 204 as image data 228. It will be appreciated that each imageframe comprises a two-dimensional array of pixels. Each pixel at leasthas corresponding photometric information (e.g., intensity, color,and/or brightness). In some embodiments, the image sensors 134, 138 mayalso be configured to capture geometric information (e.g., depth and/ordistance) corresponding to each pixel. In such embodiments, the imagesensors 134, 138 may, for example, take the form of two RGB camerasconfigured to capture stereoscopic images from which depth and/ordistance information can be derived, and/or an RGB camera with anassociated IR camera configured to provide depth and/or distanceinformation.

As will be discussed below, at least in some embodiments, certainprocesses of the method 300 are performed with respect to every imageframe, whereas other processes are only performed every so many frames(e.g., every 75 frames or every 3 seconds). As described below, this maybe defined in the form of a hyperparameter detect_every_frame having anumerical value (e.g., 75). Accordingly, in at least some embodiments,as each image frame is received and processed, the processor 200 of thevehicle computer 130 is configured to increment a frame count, which isfor example stored in the memory 204.

The method 300 continues with a step of deriving an activity vectorbased on the image frame (block 320). Particularly, the processor 200 ofthe vehicle computer 130 calculates an activity vector X_(i) for eachimage frame received from the image sensors 134, 138, where i indicatesan index of the image frame. As used herein, an “activity vector” refersto a numerical vector representing at least (i) pose of at least onepassenger in the image frame, and (ii) a motion of the at least onepassenger in the image frame. As used herein, a “pose” of a passengerrefers to a position, posture, orientation, or the like of thepassenger. Particularly, in the detailed embodiments described herein,the activity vector represents the positions of a plurality of keypoints corresponding to particular joints and body parts of eachpassenger in the image frame, as well as directions and speeds of motionof those key points.

FIG. 4 shows a logical flow diagram for a method 400 of deriving anactivity vector for an image frame. In the description of the method400, statements that a method, process, module, processor, system, orthe like is performing some task or function refers to a controller orprocessor (e.g., the processor 200) executing programmed instructions(e.g., the program instructions 208) stored in non-transitory computerreadable storage media (e.g., the memory 204) operatively connected tothe controller or processor to manipulate data or to operate one or morecomponents in the cabin monitoring system 108 and/or the vehicle 100 toperform the task or function. Additionally, the steps of the methods maybe performed in any feasible chronological order, regardless of theorder shown in the figures or the order in which the steps aredescribed. It will be appreciated that, in some embodiments, theoperations of the processor 200 described herein can be performed byother components of the vehicle 100 and/or of the cabin monitoringsystem 108, such as the vehicle ECU 112 or integrated image processorsof the sensors 134, 138, etc. Additionally, in some embodiments, theoperations of the processor 200 described herein can be performed by aremote server, such as in cloud computing system.

Given an image frame, the method 400 begins with a step of detecting keypoints for each of (e) passengers in the image frame (block 410).Particularly, the processor 200 of the vehicle computer 130 detects aplurality of key points corresponding to particular joints or body partsof each passenger in the image frame using the pose detection model 220.In at least one embodiment, the processor 200 also detects the number ofpassengers (e) in the image frame using the pose detection model 220. Inat least one embodiment, the pose detection model 220 comprises a deepneural network (DNN) which has been trained based on a corpus oftraining data (which is different from the training data discussed aboveused for training the GMM of the activity classification model 224). Theprocessor 200 executes program instructions of the the pose detectionmodel 220 with reference to a set of learned parameters, weights, and/orkernel values, which were learned during training of the pose detectionmodel 220, to detect the plurality of key points for each passenger. Inat least one embodiment, each key point is in the form of atwo-dimensional coordinate pair (x_(t), y_(t)), where x_(t) represents ahorizontal position in the image frame, y_(t) represents a verticalposition in the image frame, and t represents a time or frame number ofthe image frame. However, it will be appreciated that three-dimensionalcoordinate triplets may be also be used in the case that the imagesensors 134, 138 provide depth and/or distance information.

FIG. 5 shows an exemplary image frame 500 in which two passengers areriding in the back seat of the vehicle 100. A plurality of key points510 are identified for each of the two passengers. In the illustratedexample, the pose detection model 220 is configured to detect 25 keypoints including: (1) Right Eye, (2) Left Eye, (3) Nose, (4) Right Ear,(5) Left Ear, (6) Neck, (7) Right Shoulder, (8) Left Shoulder, (9) RightElbow, (10) Left Elbow, (11) Right Wrist, (12) Left Wrist, (13) RightHip, (14) Middle Hip, (15) Left Hip, (16) Right Knee, (17) Left Knee,(18) Right Ankle, (19) Left Ankle, (20) Right Heel, (21) Left Heel, (22)Right Big Toe, (23) Left Big Toe, (24) Right Small Toe, and (25) LeftSmall Toe. However, it will be appreciated that certain key points 510may be out of frame or occluded for a particular image frame.

In at least one embodiment, the processor 200 is configured to smoothvalues coordinate values that are predicted by the pose detection model220 by for the key points of each passenger. Particularly, the predictedcoordinate values provided by the pose detection model 220 may have someundesirable jittering between image frames due to limitations in modelperformance. To overcome such artifacts, the processor 200 is configuredto calculate the coordinate values for each key point as an average of asequence of predicted coordinate values from the pose detection model220. Particularly, the processor 200 calculates the coordinate valuesfor each key point at a time or frame number t according to theequation:

(x _(t) ,y _(t))=(Median_(k=t,t-1 . . . , t-Pose) _(smooth) (x_(k)*),Median_(k=t,t-1 . . . , t-Pose) _(smooth) (y _(k)*)),

where (x_(t)*, y_(t)*) are predicted coordinate values provided by thepose detection model 220 at a time or frame number t, and Pose_(smooth)is an integer-valued smoothing hyperparameter (e.g., 10). In otherwords, the processor 200 calculates calculate the coordinate values foreach key point as an average of the predicted coordinate values for thecurrent image frame and predicted coordinate values for a predeterminednumber Pose_(smooth) of previous image frames.

Returning to FIG. 4, the method 400 continues with a step of determiningan optical flow vector for each key point (block 420). Particularly, theprocessor 200 of the vehicle computer 130 calculates an optical flowvector for each key point representing a direction and speed of motionfor respective key point. In some embodiments, the processor 200 eachoptical flow vector for a key point as a difference between thecoordinate values for key point in the current image frame and thecoordinate values for key point in a previous image frame. Particularly,in one embodiment, the processor 200 calculates the optical flow vectorfor a key point at a time or frame number t according to the equation:

(x _(t) _(flow) ,y _(t) _(flow) )=(x _(t) −x _(t-Flow) _(smooth) ,y _(t)−y _(t-Flow) _(smooth) ),

where (x_(t) _(flow) , y_(t) _(flow) ) is the optical flow vector forthe key point (x_(t), y_(t)) at time t and Flow_(smooth) is aninteger-valued smoothing hyperparameter (e.g., 3).

FIG. 6 shows an exemplary sequence 600 of five image frames in which twopassengers are riding in the back seat of the vehicle 100. An opticalflow vector for a key point corresponding to the left ear of thepassenger on the right side of the image frames is calculated bycomparing the coordinate values of the left ear key point in a frame att=5 with the coordinate values of the left ear key point in a frame att=2.

Returning to FIG. 4, the method 400 continues with a step of, for eachpassenger, sorting key points into cells of an a×b grid based oncoordinates thereof and into an d-bin histogram for each cell based onoptical flow angles thereof (block 430). Particularly, the receivedimage frames are divided into a cells of a a×b grid, where a is aninteger-valued grid height hyperparameter (e.g., 7) and b is aninteger-valued grid width hyperparameter (e.g., 13). Each cell of thea×b grid represents a range of horizontal coordinate values and a rangeof vertical coordinate values within the image frame. In at least oneembodiment, each cell of the a×b grid has an equal size. For example,with reference to FIG. 5, the exemplary image frame 500 is divided intoa 7×13 grid of cells 520. The sequence 600 of five image frames of FIG.6 is similarly divided into grids of cells.

Additionally, a d-bin histogram (e.g., a 3-bin histogram) is defined foreach passenger for each cell of the a×b grid. Each of the d bins of ineach histogram represents a range of optical flow angles. For example a3-bin histogram might include a first bin representing a range ofoptical flow angles 0°-120°, a second bin representing a range ofoptical flow angles 120°-240°, and a third bin representing a range ofoptical flow angles 240°-360°. The optical flow angles may be withrespect to any arbitrary zero-angle, such as with respect to thehorizontal x-axis of the image frame and/or the a×b grid. It will beappreciated that the optical flow angle of an optical flow vectorrespect to the horizontal x-axis can be calculated according to theequation θ_(t) _(flow) =tan⁻¹(y_(t) _(flow) /x_(t) _(flow) ), whereθ_(t) _(flow) is the optical flow angle of an optical flow vector (x_(t)_(flow) , y_(t) _(flow) ) for the key point (x_(t), y_(t)) at time t.

The processor 200 sorts the key points for each particular passengerinto the cells of the a×b grid by comparing the coordinate values of thekey points with the ranges of values corresponding to each particularcell of the a×b grid. In other words, if the coordinate values (x_(t),y_(t)) for a key point are within the range of values that define aparticular cell of the a×b grid, then the processor 200 sorts the keypoint into that particular cell of the a×b grid. Next, the processor 200sorts the key points for each passenger in each cell of the a×b gridinto one of the bins in the respective d-bin histogram for therespective passenger for the respective cell of the a×b grid, bycomparing the optical flow angles of the key points with the ranges ofrange of optical flow angles for the respective bins of the histograms.In other words, if a key point has an optical flow angle within therange of range of optical flow angles defined by a particular bin, thenthe processor sorts the key point into that particular bin. It will beappreciated that, since there are ax b cells each having an d-binhistogram for each of the e passengers, each key point is thus sorted ina respective one of a×b×d×e different bins, depending on theircoordinate values (x_(t), y_(t)) and their optical flow angle θ_(t)_(flow) .

The method 400 continues with a step of calculating a numerical valuefor each histogram bin of each cell for each passenger, resulting in anactivity vector for the given image frame (block 440). Particularly, theprocessor 200 calculates, for each bin of each histogram in each cellfor each passenger, a numerical value equal to a sum of the magnitudesof the optical flow vectors of the key points that have been sorted intothe respective bin. More particularly, the processor 200 calculates themagnitude of the optical flow vector for each key point. It will beappreciated that the magnitude of an optical flow vector can becalculated according to the equation M_(t) _(flow) ²=x_(t) _(flow)²+y_(t) _(flow) ², w here M_(t) _(flow) is the magnitude of an opticalflow vector (x_(t) _(flow) , y_(t) _(flow) ) for the key point (x_(t),y_(t)) at time t. Finally, the processor 200 calculates the numericalvalue for each bin as a sum of the magnitudes of the optical flowvectors for the key points that were sorted into the respective bin.These calculated numerical values form an activity vector X_(i) withdimensions a×b×d×e for the image frame, where i indicates an index ofthe image frame. It will be appreciated that the magnitude of thecalculated numerical values scale with the amount of activity in theimage frame in the respective region and direction defined by therespective cell and histogram bin. In this way, the activity vectorX_(i) encodes the movements and/or activity of the two passengers withinthe image frame in a numerical form that can be more easily evaluated.

FIG. 7 shows a further exemplary image frame 700 in which a passenger ispushing another passenger. FIG. 8 shows a graph 800 illustrating theactivity vector calculated based on the exemplary image frame 700. Inthe graph 800, the cells 810 correspond to cells 710 of the exemplaryimage frame 700. In each cell 810 of the graph 800, a 3-bin histogram isshown for each of passenger. Particularly, the optical flow vectors andkey points for passenger on the right hand side of the image frame 700are represented with solid-black histogram bins 820 in the graph 800.Conversely, the optical flow vectors and key points for passenger on theleft hand side of the image frame 700 are represented withdiagonal-hatched histogram bins 830 in the graph 800. The heights ofeach histogram bin correspond to the calculated numerical values of theactivity vector X_(i). As can be seen, there is only minimal overlap ofkey points for the two the passengers (i.e., only one cell showshistograms for both passengers). Additionally, as can be seen, the cellscorresponding the to the left arm of the passenger on the on the lefthand side of the image frame 700 show diagonal-hatched histogram binshaving comparatively large heights, indicating comparatively highmagnitude motion (i.e. fast motion).

Returning to FIG. 3, the method 300 continues with a step of classifyingthe image frame into a cluster having a highest posterior probability,based on the activity vector (block 330). Particularly, for each imageframe, the processor 200 determines which of a plurality of learnedcluster components C_(i) the activity vector X_(i) most likelycorresponds. More particularly, the processor 200 executes programinstructions of the activity classification model 224, with reference toa plurality of learned cluster components C_(i), to classify theactivity vector X_(i) as most likely belonging to a particular learnedcluster component C_(i). In other words, the cluster component C_(i) istreated as a latent variable describing the class of activityrepresented in the image frame and is predicted based the measuredactivity vector X_(i).

As noted above, the activity classification model 224 comprises aGaussian Mixture Model (GMM) that defines a plurality of clustercomponents C_(i) that correspond to normal passenger behavior. Thecluster components C_(i) each comprise a normal distributionN(μ_(c),Σ_(c)) over the dimensions a×b×d×e (i.e., the same dimensions asthe activity vectors X_(i)), where μ_(c) is a cluster center and/ormedian value having dimensions a×b×d×e and Σ_(c) is a covariance matrixhaving dimensions a×b×d×e. The GMM of the activity classification model224 is formed by k different cluster components C_(i). In other words,given a cluster component, the per-frame activity vector is fromp-dimensional multivariate normal:

C·Categorical(p ₁ , p ₂ , . . . p _(K))

X _(i) |C _(i) =c˜N(μ_(c),Σ_(c))

where the variable C is a categorical distribution with K differentcategories, p₁, p₂, . . . p_(K) are density functions with dimensionsa×b×d×e that indicate the chance of the variable C to take a particularvalue c, and N(μ_(c),Σ_(c)) is the normal distribution for a particularvalue c.

Based on the activity vector X_(i) for the particular image frame, theprocessor 200 classifies the image frame into the cluster componentC_(i) with the highest posterior probability according to the equation:

c _(i)=argmax_(k) Pr(C _(i) =k|X _(i)).

In other words, for each value i=1, . . . , k, the processor 200calculates the posterior probability Pr(C₁=k|X_(i)), indicating aprobability that the activity vector X_(i) belongs to the particularcluster component C_(i). The processor classifies the activity vectorX_(i) as belonging to the cluster component C_(i) having the highestposterior probability Pr(C_(i)=k|X_(i)). The cluster component C_(i) towhich the activity vector X_(i) belongs is denoted herein as c_(i). Inat least one embodiment, the processor 200 stores the determined clustercomponent c_(i) to which the activity vector X_(i) most likely belongsin the memory 204.

As suggested above, prior to deployment of the cabin monitoring system104, the plurality of cluster components C_(i) are learned based onunlabeled training data in the form of video of passengers riding in thecabin 108 of the vehicle 100. Particularly, a large set of trainingactivity vectors are derived from the image frames from the trainingvideo in the manner described above with respect to FIG. 4. The largeset of training activity vectors X_(i) is used to derive the GMM havingk different cluster components C_(i) that best model the large set oftraining activity vectors X_(i). The unknown parameters μ_(c) and Σ_(c)for each cluster component C_(i) are estimated using theExpectation-Maximization Algorithm.

Additionally, it will be appreciated that GMMs require that the numberof cluster components k be pre-specified. In at least one embodiment,the number of cluster components k is selected by Akaike InformationCriteria (AIC). AIC is defined as:

${{AIC} = {{2\ln P} - {\ln\; L}}},\begin{matrix}{L = {f\left( {\left\{ X_{i} \right\}_{i = 1}^{n};\left\{ {\mu_{l},{\Sigma_{l}c_{l}}} \right\}_{l = 1}^{K}} \right)}} \\{= {\prod_{i = 1}^{n}{f\left( {X_{i};\left\{ {\mu_{l},{\Sigma_{l}c_{l}}} \right\}_{l = 1}^{K}} \right)}}} \\{= {\Pi_{i = 1}^{n}{f\left( {{{\left. X_{i} \middle| C_{i} \right. = c};\mu_{c}},\Sigma_{c}} \right)}{f\left( {{C_{i} = c};p_{c}} \right)}}} \\{{= {\Pi_{i = 1}^{n}{N\left( {{X_{i};\mu_{c}},\Sigma_{c}} \right)}p_{c}}},}\end{matrix}$

where P is the number of unknown parameters (i.e., μ_(l), Σ_(l), andc_(l), where l=1, . . . , K) to be estimated and L is the likelihoodfunction or, in other words, the density at observed training activityvectors X_(i), i=1, . . . n, where n is the total number of trainingactivity vectors X_(i).

A smaller AIC indicates better fit of the model while penalizing the useof complex model, measured by the number of unknown parameters P. In oneembodiment, the AIC is calculated for a predetermined range of valuesfor k (e.g., k=1, . . . , 20) and the value of k resulting in the lowestAIC is used for deriving the GMM of the activity classification model224.

In at least one embodiment, this training process is uniquely performedfor different numbers of passengers using unlabeled training data in theform of video of the respective number of passengers riding in the cabin108 of the vehicle 100. Particularly, respective pluralities of clustercomponents C_(i) may be learned for a single passenger riding alone, fortwo passengers riding together, for three passengers riding together,and so on, up to some reasonable upper limit on the number of passengersexpected to ride in a particular area of the cabin 108 that is in viewof an image sensor.

The method 300 continues with a step of determining a posterior densityfor the image frame (block 340). Particularly, once the clustercomponent c_(i) to which the activity vector X_(i) most likely belongsis determined, the processor 200 calculates the posterior densityaccording to the equation:

posterior density_(i) =f(X _(i) |C _(i) =c _(i)),

where f( ) is the probability density function of the GMM, which isevaluated given the activity vector X_(i) and the determined clustercomponent c_(i). In at least one embodiment, the processor stores theposterior density_(i) for the image frame in the memory 204

As described below, the image frame can be considered an anomaly or asincluding abnormal passenger behavior if the determined posteriordensity for the image frame is below a predefined threshold. In thisway, the processor 200 can detect abnormal passenger behavior on aframe-by-frame basis by comparing the posterior density for each imageframe with the predefined threshold. However, it is generally notnecessary to detect whether anomaly arises at every frame (e.g., every1/25=0.04 second), because the abnormal behavior situation would notchange at such a high frequency. Thus, in at least one embodiment, theprocessor 200 instead detects abnormal passenger behavior only every somany frames based on an average posterior density over several frames.

The method 300 repeats the steps 310-340 to determine posteriordensities for a sequence of image frames until the frame count is equalto a threshold number of frames (block 350). Particularly, as notedabove, as each frame is received, the processor 200 increments aframe_count. As each image frame is received, the processor 200 repeatsthe processes of deriving an activity vector X_(i), determining thecluster component c_(i) to which the activity vector X_(i) most likelybelongs, and calculating a posterior density_(i) for the image frame,until the frame_count is equal to the hyperparameter detect_every_frame(e.g., 75, such that, at 25 frames per second, abnormal behavior isdetected every 3 seconds).

The method 300 continues with a step of checking whether an averageposterior density for the sequence of image frames is less than athreshold (block 360). Particularly, the processor 200 calculates anaverage of the posterior density_(i) for all of the image framesreceived since the frame_count was last reset and abnormal behaviordetection was last performed, and compares the average with apredetermined anomaly threshold. In other words, the processor 200evaluates the equation:

$\frac{\Sigma_{i = {t - {{{detect}\_{every}}{\_{frame}}}}}^{t}{f\left( {\left. X_{i} \middle| C_{i} \right. = c_{i}} \right)}}{{detect\_ every}{\_ frame}} < {{threshold}.}$

If the average posterior density is less than the threshold, the method300 continues with detecting abnormal passenger behavior (block 370).Particularly, in response to the average posterior density being lessthan the predetermined anomaly threshold, the processor 200 detects thatabnormal passenger behavior has occurred. In at least one embodiment, inresponse to detecting abnormal passenger behavior, the processor 200operates the transceivers 212 to transmit an anomaly notificationmessage to a remote server, such as a cloud backend or remote database.The anomaly notification message may include the image frame and/or theactivity vector X_(i) with respect to which the abnormal passengerbehavior was detected.

The remote server may, for example, be accessible by an operator of anautonomous taxi service or other similar autonomous vehicle service orshared vehicle service and may interface with an external cloud serviceassociated with the service. In one embodiment, the remote server isconfigured to notify (e.g., via email or the like) the operator inresponse to abnormal behavior being detected. In other embodiments, theoperator can access the relevant image data and/or abnormal behaviorevent data stored on the remote server via a web portal.

In further embodiments, in response to detecting abnormal passengerbehavior, the processor 200 may operate a speaker or display screen (notshown) arrange within the cabin 108 of the vehicle 100 to display, play,or otherwise output an alert or warning to the passengers, for exampleurging the passengers to cease the abnormal behavior.

Regardless of whether the average posterior density is less than thethreshold, the method 300 continues with a step of resetting the framecount before repeating the method 300 entirely (block 380).Particularly, after the abnormal behavior detection, the processor 200resets the frame_count to zero and repeats the processes of receivingimage frames, deriving activity vectors X_(i), determining the clustercomponents c_(i) to which the activity vectors X_(i) most likely belong,and calculating a posterior density_(i) for each image frame, until theframe_count is equal to the hyperparameter detect_every_frame, beforeperforming the abnormal behavior detection again.

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe disclosure are desired to be protected.

What is claimed is:
 1. A method for detecting abnormal passengerbehavior in a vehicle, the method comprising: receiving, with aprocessing system, a first image frame of at least one passenger in acabin of the vehicle from an image sensor; determining, with theprocessing system, based on the first image frame, a first numericalvector representing a pose and a motion of the at least one passenger inthe first image frame; and detecting, with the processing system, basedon the first numerical vector, abnormal passenger behavior in the firstimage frame using a mixture model having a plurality of clustercomponents representing normal passenger behaviors.
 2. The method ofclaim 1, the determining the first numerical vector further comprising:determining, with the processing system, a respective plurality of keypoints for each of the at least one passenger, each key point includinga coordinate pair corresponding to a position of a respective joint orbody part of the at least one passenger within the first image frame. 3.The method of claim 2, the determining the respective plurality of keypoints for each of the at least one passenger further comprising:determining, with the processing system, the coordinate pair of each keypoint of the respective plurality of key points for each of the at leastone passenger as an average position of the respective joint or bodypart of the at least one passenger over multiple image frames includingthe first image frame and at least one previous image frame.
 4. Themethod of claim 2, the determining the first numerical vector furthercomprising: determining, with the processing system, for each key pointof the respective plurality of key points for each of the at least onepassenger, an optical flow vector indicating a motion of the respectivekey point in the first image frame with respect to at least one previousimage frame.
 5. The method of claim 4, the determining the optical flowvector further comprising: determining, with the processing system, adifference between the coordinate pair of the respective key point inthe first image frame and a previous coordinate pair of the respectivekey point in a previous image frame.
 6. The method of claim 4, thedetermining the first numerical vector further comprising: sorting, withthe processing system, each key point of the respective plurality of keypoints for each of the at least one passenger into a respective cell ofa two-dimensional grid of cells based on the coordinate pair of therespective key point, where each respective cell of the grid correspondsto a range of coordinates within the first image frame; sorting, withthe processing system, each key point sorted into each respective cellof the grid into a respective bin of a respective histogram for each ofthe at least one passenger based on an optical flow angle of the opticalflow vector of the respective key point, where each bin of therespective histogram for each of the at least one passenger correspondsto a range of optical flow angles; determining, with the processingsystem, a numerical value for each bin of the respective histogram foreach of the at least one passenger as a sum of optical flow magnitudesfor the optical flow vectors of each key point sorted into therespective bin; and forming, with the processing system, the firstnumerical vector with the numerical value for each bin of the respectivehistogram for each of the at least one passenger.
 7. The methodaccording to claim 6, wherein the first numerical vector has dimensionsa×b×d×e, where a×b are dimensions of the grid, d is a number of bins inthe respective histogram for each of the at least one passenger, and eis a number of passengers of the at least one passenger.
 8. The methodof claim 1, the detecting abnormal passenger behavior furthercomprising: determining, with the processing system, for each clustercomponent of the plurality of cluster components of the mixture model, aposterior probability that the first numerical vector belongs to therespective cluster component; and classifying, with the processingsystem, the first image frame as belonging to a first cluster componentof the plurality of cluster components of the mixture model having ahighest posterior probability.
 9. The method of claim 8, the detectingabnormal passenger behavior further comprising: determining, with theprocessing system, a first posterior density based on the firstnumerical vector and the first cluster component of the plurality ofcluster components of the mixture model.
 10. The method according toclaim 9, the detecting abnormal passenger behavior further comprising:comparing, with the processing system, the first posterior density witha predetermined threshold; and detecting the abnormal passenger behaviorin the first image frame in response to the first posterior densitybeing less than the predetermined threshold.
 11. The method according toclaim 9, the detecting abnormal passenger behavior further comprising:determining, with the processing system, an average posterior densityover multiple image frames including the first image frame and at leastone previous image frame; comparing, with the processing system, theaverage posterior density with a predetermined threshold; and detectingthe abnormal passenger behavior in the first image frame in response tothe average posterior density being less than the predeterminedthreshold.
 12. The method according to claim 1, wherein the plurality ofcluster components are learned using unlabeled training data, theunlabeled training data including a corpus of video of at least onepassenger riding in the vehicle.
 13. The method according to claim 1further comprising: transmitting, with a transceiver, a message to aremote server in response to detecting abnormal passenger behavior. 14.The method according to claim 1 further comprising: outputting, with aspeaker or display screen, an alert to the at least one passenger inresponse to detecting abnormal passenger behavior.
 15. A system fordetecting abnormal passenger behavior in a vehicle, the systemcomprising: an image sensor configured to generate and output imageframes of at least one passenger in a cabin of the vehicle; a processingsystem operably connected to the image sensor and including at least oneprocessor, the processing system configured to: receive a first imageframe of at least one passenger in a cabin of the vehicle from an imagesensor; determine, based on the first image frame, a first numericalvector representing a pose and a motion of the at least one passenger inthe first image frame; and detect, based on the first numerical vector,abnormal passenger behavior in the first image frame using a mixturemodel having a plurality of cluster components representing normalpassenger behaviors.
 16. The system of claim 15, the processing systemfurther configured to, in the determination of the first numericalvector: determine a respective plurality of key points for each of theat least one passenger, each key point including a coordinate paircorresponding to a position of a respective joint or body part of the atleast one passenger within the first image frame; and determine for eachkey point of the respective plurality of key points for each of the atleast one passenger, an optical flow vector indicating a motion of therespective key point in the first image frame with respect to at leastone previous image frame.
 17. The system of claim 16, the processingsystem further configured to, in the determination of the firstnumerical vector: sort each key point of the respective plurality of keypoints for each of the at least one passenger into a respective cell ofa two-dimensional grid of cells based on the coordinate pair of therespective key point, where each respective cell of the grid correspondsto a range of coordinates within the first image frame; sort each keypoint sorted into each respective cell of the grid into a respective binof a respective histogram for each of the at least one passenger basedon an optical flow angle of the optical flow vector of the respectivekey point, where each bin of the respective histogram for each of the atleast one passenger corresponds to a range of optical flow angles;determine a numerical value for each bin of the respective histogram foreach of the at least one passenger as a sum of optical flow magnitudesfor the optical flow vectors of each key point sorted into therespective bin; and form the first numerical vector with the numericalvalue for each bin of the respective histogram for each of the at leastone passenger.
 18. The system of claim 15, the processing system furtherconfigured to, in the detection of the abnormal passenger behavior:determine, for each cluster component of the plurality of clustercomponents of the mixture model, a posterior probability that the firstnumerical vector belongs to the respective cluster component; classifythe first image frame as belonging to a first cluster component of theplurality of cluster components of the mixture model having a highestposterior probability; and determine a first posterior density based onthe first numerical vector and the first cluster component of theplurality of cluster components of the mixture model.
 19. The system ofclaim 18, the processing system further configured to, in the detectionof the abnormal passenger behavior: determine an average posteriordensity over multiple image frames including the first image frame andat least one previous image frame; compare the average posterior densitywith a predetermined threshold; and detect the abnormal passengerbehavior in the first image frame in response to the average posteriordensity being less than the predetermined threshold.
 20. The system ofclaim 15 further comprising: a transceiver operably connected to theprocessing system, wherein the processing system is configured tooperate the transceiver to transmit a message to a remote server inresponse to detecting abnormal passenger behavior.